Process for the manufacture of boron carbide molding powders



United States Patent 3,137,584 PROCESS FOR THE MANUFAQT OF BORON CARBEDEMOLDDIG POWDERS Guy H. Fetterley, deceased, late of Chippawa, Ontario,Canada, by James A. Pollard, executor, St. Catharines, Ontario, Canada,assignor to Norton Company, Worcester, Mass, a corporation ofMassachusetts No Drawing. Continuation of application Ser. No. 3%,941,Nov. 23, 1953. This application May 22, 1961, Ser. No. 112,174

1 Claim. (Cl. 10643) This application is a continuation of pendingapplication Serial No. 393,941, filed November 23, 1953, now abandoned.

The object of this invention is to provide a molding powder that meetsthe following requirements:

(a) It can be made economically and in commercial quantities from thecrude boron carbide prepared according to US. Patent No. 2,155,682.

(1)) It can be molded into solid bodies by standard hot pressing methodswith minimum rejections foreither low density or breakage in the mold.

(c) The molded pieces will be extremely hard and durable because theyare high in density (low in porosity), free from excessive grain growthand contain a minimum of free carbon which usually appears as graphiteinclusions.

Other objects will be in part obvious or in part pointed outhereinafter.

According to US. Patent No. 1,897,214, issued on application of the lateRaymond R. Ridgway, boron carbide of the best quality was determined tobe B C. According to Ridgways later US. Patent No. 2,027,786, whichdescribes the molding of boron carbide, powders having as nearly aspossible the formula 13 C should be used. The method of making boroncarbide of the formula 3 C is fully described in his US. Patent No.2,155,- 682, and a furnace suitable for molding this material into solidbodies is described in his US. Patent No. 2,125,- 588.

While the procedures described by Ridgway were used successfully on alaboratory and small commercial scale, and produced molded boron carbidepieces of excellent quality, these procedures also led to a number ofdifficulties which are discussed below:

Crude boron carbide, made according to US. Patent No. 2,155,682, wasfound to have a variable B/C ratio, here defined as the number of atomsof boron per atom of carbon. This is what is called a mole ratio, inthis case the boron to carbon mole ratio in the specification referredto as the B/ C ratio for short. After each run the furnace was unloadedand the dense, crystalline, wellreacted portion was sorted out asproduct, the remainder being returned to the furnace as part of the rawmaterial for subsequent runs. When an average sample of this product wasanalyzed it might show a B/C ratio from 3.5 to 4.5 or even higher,depending on normal variations in the raw materials and furnacingconditions, not all of which can be controlled under conditions of fullscale production without incurring excessive costs. If it were thennecessary to reject, as raw material for making molding powder, all lotsthat did not show a B/C ratio of exactly 4.0, the cost of molding powderwould be prohibitively high. Recent studies of the boron-car bon systemindicate that this variation in B/ C ratio is to be expected, because2,0 can hold considerable quantities of boron in solid solution whenexcess boron is present, and can also dissolve carbon at hightemperatures when excess carbon is present. The excess carbon is almostall precipitated as included graphite when the 3,0 cools, but appears inthe analysis because it is intimately mixed with the 3 C.

Molding powder of theoretical composition, having four atoms of boron toeach atom of carbon was found to be ditiicult to mold with high yieldsof satisfactory pieces. Even with the most precise possible control oftemperature and pressure in the molding process, many of the finishedpieces would be undesirably high in porosity, or would be found to bebroken when they were removed from the mold. It is now believed thatthese difiiculties were caused by-random fluctuations in the B/ C ratioof individual particles of powder in contact with the wall of thegraphite mold. These particles having a B/C ratio in excess of 4.0reacted with the mold wall at the moldingtemperature and adhered verytightly to the wall. Becauseoffthis adherence, the powder could notslide freely along the wall as it mustif it is to be compacted to a highdensity.

In US. Patent No. 2,027,786, it is stated that the molding powder shouldbe fine enough'to pass a 200 mesh screen, but no lower limit wasplacedon, the particle size. It was later found, however, that powders havingall the particles liner than ten microns were very difficult to mold,because isolated crystals in the molded piece would grow to large size,easily visible to the naked eye on the broken surface. This. conditionis highly undesirable, since the hardest and most durable molded piecesshow a microstructure in which practically none of the individualcrystals exceed 50 microns in diameter. Usually, when such coarsecrystallization occurred in the molding process, the molded pieces werevery weak and were frequent- 1y found to be broken when removed from themold.

Powders prepared according to US. Patent No. 2,027,- 786 were found tobe fluffy and hard to handle. Dust losses were high and the powders werehard to pack into a mold. Excessively long molds were required toaccommodate the powder after loading but before heating, in creasing thecost of molds. In addition, labor costs were increased bythe amountofeffort required to pack the powder in the mold. V

The difficulties described above were overcome by developing thefollowing techniques:

Itwas found that crude lots having B/C ratios from 3.65 to 4.05 could beused to make a satisfactory grade of molding powder, providing thefinalpowder was made up by blending analyzed sub-lots in such a way thatthe average B/ C ratio fell within a narrow range, discussed in the nextparagraph. This discovery made it possible to accept for the manufactureof molding powder a large proportion of thelots of crude boron carbidethatwere produced commercially.

As noted above, it was found that powder within an average B/C ratio of4.0 or higher was diflicult to mold. Furthermore, if the B/C ratio waslower than 3.8 flakes of graphite appeared in the molded pieces,reducing their hardnessfand strength. A critical range of B/C ratios wasfound, from 3.8 'to 3.9, such that the powder was much easier to moldthan that having a B/C ratio of 4.0

and-at the'same time produced molded pieces substantially freefromgraphite inclusions and havinga degree in the molded pieces, but only atthe cost of greatly increased difliculty of molding.

Excessive flufliness, low bulk density and resulting han-' dlingdifliculties were overcome by adding to the finished powder 1% of waterwitha very small amount of Wetting agent such as Aerosol OT. Aerosol OTis the trade name of a commercially available wetting agent, which is asodium salt of benzene sulphonic acid.

The molding powder is made according to the invention, by following thetechnology of the aforesaid U.S. Letters Patent, Nos. 1,897,214 and2,155,682 and thereby producing a number of lots of crude boron carbide.An average sample of each lot is analyzed chemically, and for each lotthe B/C ratio and total percentage of B-l-C are noted. Any lot havingB+C less than 98% is rejected, and also any lot having a B/ C less than3.65 or more than 4.05 is rejected. The rejected lots can be used forpurposes other than the preparation of molding powder, such as themanufacture of abrasive grains or use as a raw material for thesynthesis of other boron compounds. In this way profitable use can bemade of practically all the crude boron carbide that is made in thecrude furnaces, and at the same time get an ample supply of raw materialfor the manufacture of molding powder.

A typical manufacturing operation to make molding powder from the crudeboron carbide follows. Note that the quantities given in the example aredetermined in practice by convenience in handling and by the capacity ofthe equipment on hand.

Each lot of crude boron carbide is passed through a jaw crusher to breakthe lumps down to about inch and finer. Then this material is passedthrough a roll crusher and 24 mesh wire screen. The roll crusher andscreen are operated in closed circuit, the oversize returning from thescreen to the crusher, so that all the product eventually passes throughthe screen.

Then a 70 lb. batch is selected, which may be all taken from one of theabove-mentioned crude lots or may be made up from two or more of them.At this stage it is convenient to select lots which will yield a normalbalance of B/ C ratios in the inventory of molding powder. Thus if theinventory contains more powder having a B/C ratio below 3.85 than aboveit, lots with a 13/0 ratio above 3.85 are selected until the inventorycomprises lots that are evenly balanced in B/C ratio about the averageof 3.85.

The 70 lb. batch is placed in a steam-jacketed kettle with enough waterto cover the solid, boil the water slowly with occasional stirring fortwo hours, and allow it to settle for one hour. The hot water removesboric acid, graphite, wood chips, etc. which are skimmed oif the topduring the washing.

After draining off excess water, the 70 lbs. of washed crude boroncarbide is transferred to a ball mill with about 7 gallons of water. Themill consists of a steel drum about inches long and 30 inches indiameter and contains 500 lbs. of /2 inch steel balls. After sealing,the mill is rotated for 12 hours at 30 r.p.m. About 6 lbs. of balls isused up in this operation.

The mill is provided with a port about 12 inches square. This port issealed during milling by a steel plate. At the end of 12 hrs. ofmilling, this plate is removed and a heavy steel screen, having openingsA; inch square, is bolted in its place. A shallow steel tank on castersis rolled under the mill, which is then turned so that the slurry runsout into the tank but the balls are retained by the screen. The mill isfinally flushed out with 3 gallons of water.

The next steps are acid treatment and water washing to remove the ironadded by the ball milling. The shallow steel tank, containing theslurry, is rolled to the acidtreating tank, which is an open-toppedvertical cylinder 30 inches in diameter and 45 inches high, made ofleadlined steel. The slurry is pumped into this tank through a 66 meshscreen, oversize caught on the screen being returned to the ball millwith the next batch.

The slurry is stirredrapidly in the bottom of the tank with amotor-driven stirrer having a long shaft, and lbs. of 96% sulphuric acidis added slowly. This operation must be done carefully, since the tankfills with foam, and a large volume of hydrogen is evolved. The hydro- 4gen is inflammable and is contaminated with other gases, some of whichare poisonous, so an efficient hood must be provided to exhaust thesegases from the working space.

At the end of 3 hrs., about 75 gallons of water is added, filling thetank to of its capacity. The stirrer is run for 12 hrs. more, thenturned off, and the solids are allowed to settle for 4 hrs. The clearliquid is siphoned off and discarded, leaving 3-4 inches of thick slurryin the bottom. About 10 gallons of water and 20 lbs. of 96% sulphuricacid are added, and steam is passed into the slurry with stirring toheat it to boiling for hr. About 75 gallons of water is-added, and theresulting dilute slurry is stirred for 12 hrs., then allowed to settlefor 12 hrs. The slight acidity promotes settling. The clear liquid issiphoned off and discarded as before.

About 10 gallons of water is again added to the thickened slurry. Whilethe solids are kept in suspension by the stirrer, the liquid mixture ispumped into the filter press and washed with water for two hours. Therate of delivery of slurry and wash water to the press is determined bythe pump, which maintains a practically constant delivery of two gallonsper minute. Compressed air is then passed through the filter cake fortwo hours T to dry it partially.

The filter press is dismantled and the filter cake removed and brokenup. The partly dried material is placed in flat trays and loaded into avacuum dryer. It is heated to about C. with steam under a vacuum ofabout 28 ins. of mercury for 3 hrs.

The lumpy product from the drying process is broken down to a finepowder by a light milling in a small buhr mill.

By repeating this process many times a stock of molding powder batches,each weighing about 70 lbs. is accumulated. Since all the raw materialswere selected to have a B/ C ratio of between 3.65 and 4.05, thesebatches will also have B/C ratios in this range. A sample of each batchis analyzed for boron, carbon, and iron extractable with hothydrochloric acid. Each batch must show B+C at least 99%, B/C ratiobetween 3.65 and 4.05, and extractable iron not more than about 0.1%.Rejections are unusual at this stage of the process, but occuroccasionally. Each batch is stored separately and a record of its B/Cratio is kept for use in selecting powder for blending.

The molding powder is now finished as far as inventory is concerned andmay be stored indefinitely. One further operation is necessary before itcan be used for molding, and this operation is preferably performedshortly before the powder is to be used. Certain minor advantages accruefrom this manner of operation, since it reduces the danger of particlesize segregation from handling, minimizes variations in moisture contentfrom evaporation, and maintains the inventory in a more flexiblecondition.

To make a 100 lb. lot of molding powder selection of weighed amounts ofthe batches described above are made in such a Way that the average B/Cratio of the lot is between 3.80 and 3.90, as close to 3.85 as isconvenient. Then the entire 100 lbs. of powder is placed in arubberlined ball mill and thereto is added 1 lb. of Water to which hasbeen added one drop of 10% solution of Aerosol OT wetting agent. In themill are placed 16 lbs. of 1 /2 inch diameter rubber balls, it issealed, and rotated slowly for 2 hrs. to insure complete mixing of thepowder. The amount of water added to the powder is not critical, butless than /2% is not very efi'ective and over 2% is unnecessary.Preferably the addition is limited to the range /2% to 2% of the weightof powder.

When the mill is opened powder is packaged in paper bags, 5 lbs. to abag, to minimize particle size segregations, and stored in sealed drumsto prevent evaporation of the water that was added.

Boron carbide molding powder having thus been prepared, molded pieces ofvarious shapes can be produced by molding in the apparatus described inRidgways U.S. Patent No. 2,125,588 and in his joint U.S. Patent withBruce L. Bailey, No. 2,150,884. The technology for molding is describedin their U.S. Patent No. 2,027,786. Sand blasting nozzles and othernozzles, plug gages and other articles can readily be made. For specialmolding apparatus, see U.S. Patent to G. R. Watson, No. 2,535,180, andU.S. Patent to G. H. Fetterley and J. W. Knowlton, No. 2,522,046. Usingthe powder prepared according to my invention the pieces contain littlefree carbon, have long Wear and molding is easy, with few cracked piecesresulting. In short the molding powder herein described is far superiorand makes fewer cracked pieces than molding powder which is strictly B Cor outside the B/C ratio 3.80 to 3.90.

It will thus be seen that there has been provided by this invention aprocess for the manufacture of boron carbide molding powder in which thevarious objects hereinbefore set forth togeher with many thoroughlypractical advantages are successfully achieved. As various possibleembodiments might be made of the above invention and as the art hereindescribed might be varied in various parts, all without departing fromthe scope of the invention, it is to be understood that all matterhereinbefore set forth is to be interpreted as illustrative and not in alimiting sense.

What is claimed is:

A process for making boron carbide molding powder from masses of reactedboric acid and carbon, comprising selecting from said masses only thosemasses which show analyses of over 98% boron carbide and a B/ C moleratio of from 3.65 to 4.05, washing said selected masses to remove boricacid, carbon and other impurities, separately ball milling each of saidmasses, said ball milling being continued for a time period and underconditions to prt duce a particle size distribution within each mass inwhich to falls within a range of from 5 to 50 microns with less than 12%of the particles being under 5 microns in size and wherein there are noparticles larger than microns, subjecting separately each milled mass toan acid wash by stirring a slurry of the milled particles with slowadditions of an acid solution, washing and then drying the acid treatedmasses, selecting from such independently milled acid treated and driedmasses only those masses which upon analysis show a presence of at least99% boron carbide and which respectively have a B/ C mole ratio of from3.65 to 4.05 and an iron content of not more than 0.1%, breaking downall of the said finally selected and dried masses to fines and blendingportions of said masses having different B/C ratios and including in theblend portions of some of said masses which are outside the range of theB/ C mole ratio of between 3.80 to 3.90, said mass portions beingselected in amounts calculated to produce an ultimate mixture of moldingpowder fines having a B/C mole ratio of between 3.80 to 3.90 adapted forthe production of hot pressed molded products having little free carbon,that are long wearing and have few flaws therein.

References Cited in the file of this patent UNITED STATES PATENTS966,399 Higgens Aug. 2, 1910 1,951,133 DeBats Sept. 14, 1931 2,027,786Ridgway Ian. 14, 1936 2,529,333 Finlay Feb. 6, 1956 FOREIGN PATENTS1,116,480 France Feb. 6, 1956

